The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Manufacturers of internal combustion engines are continually developing new engine control strategies to satisfy customer demands and meet various regulations. One such engine control strategy comprises operating an engine at an air/fuel ratio that is lean of stoichiometry to improve fuel economy and reduce greenhouse gas emissions. Such engines include both compression-ignition (diesel) and lean-burn spark-ignition engines. When an engine operates in a region lean of stoichiometry, combustion temperatures typically increase, leading to increased NOx emissions. One proposed type exhaust aftertreatment system and control strategy for managing and reducing NOx emissions includes injection of a reductant into an exhaust gas feedstream upstream of a hydrocarbon-selective catalytic reduction (‘HC-SCR’) catalytic device to cause a reduction of NOx exhaust gases to nitrogen and oxygen across the HC-SCR device, among other gases.
Currently, proposed systems for injecting reductants for selective catalyst reduction of NOx require additional hardware and control schemes to accomplish such operation. An example of such a system includes injection of engine fuel into the exhaust stream. The effectiveness of this method decreases significantly at an exhaust temperature below 400° C. at the point of injection, and requires an adequate residence time of the reductant in the exhaust feedstream. Another example includes injection of ammonia into the exhaust feedstream upstream of a reduction catalyst. There are infrastructure-related problems associated with storage, transport, and handling of ammonia for a large fleet. Furthermore, un-reacted ammonia may pass through the SCR and be exhausted into the atmosphere. Another example comprises use of urea as a reductant for selective catalytic reduction. Such a system requires replenishing urea on-board the vehicle, which requires an infrastructure to distribute the urea. Furthermore, the freezing point of the urea solution is −12° C., presenting a problem for its use in cold climates. Another example comprises use of NOx storage catalysts. These catalysts can be effective, but they typically require a large catalyst volume and a substantial mass of expensive platinum-group metals (e.g., Pt, Pd, and Rh) in combination with extremely low sulfur fuel and periodic intrusive operation of the engine to regenerate the catalyst, thus reducing the effective fuel economy of the vehicle.
Hydrocarbon-selective catalytic reduction (HC-SCR) is a technology for reducing emissions of nitrogen oxides in lean exhaust, such as diesel exhaust. One of the significant challenges of implementing HC-SCR is to develop a system that allows sufficient reduction of NOx emissions over the range of exhaust temperatures found in diesel engines at the speeds and loads experienced during typical, every-day operation. Furthermore, it is desirable that the hydrocarbon reductant be present on the vehicle. One source of reductant is the exhaust itself, but typical hydrocarbons present in engine exhaust are generally less active than diesel fuel. The use of diesel fuel and selected diesel fuel-component hydrocarbons as the reductant has been explored by several investigators. Results indicate that the NOx reduction efficiency of HC-SCR catalysts can be greatly improved if appropriate active hydrocarbon species are used. The most effective active species appear to be oxygenated hydrocarbons, such as acetaldehyde and formaldehyde. These oxygenated species are produced by low-temperature oxidation of long, straight-chain alkane hydrocarbons of the type that are present in diesel fuel. Also, long, straight-chain alkenes are produced by low-temperature oxidation of these same alkanes, and have been found to be more effective reductants than the long, straight-chain alkanes in some studies.
A system that results in adequate reduction of NOx emissions over a range of exhaust temperatures typically found in diesel engines at the speeds and loads experienced in every-day driving, i.e., 200° C.-500° C., is desirable. A number of hydrocarbons have been shown to be effective for the reduction of NOx over silver-alumina (Ag/Al2O3) and barium-yttria (BaY) zeolite catalysts, including long straight-chain alkane hydrocarbons, alkene hydrocarbons, and diesel fuel, as well as alcohols and aldehydes.
Hydrocarbons present in diesel and gasoline engine exhaust such as methane, ethane, propene, and propane require high temperatures for adequate NOx conversion, and are generally not suitable for HC-SCR. Long straight-chain alkane hydrocarbons such as n-octane and n-decane that are present in diesel fuel require somewhat lower temperatures for adequate NOx conversion compared to the lighter hydrocarbons. This observation has led to the use of diesel fuel and long straight-chain alkane hydrocarbons as the NOx reductant in a number of studies. These studies have shown, however, that diesel fuel as well as long, straight-chain hydrocarbons are only adequate above a catalyst temperature of about 300° C. (573 K) and low catalyst space velocity. Further work has shown that alcohols and aldehydes can reduce NOx at catalyst temperatures as low as 200 to 250° C. (473 K to 523 K). Long, straight-chain alkene hydrocarbons have been found to be effective at temperatures as low as 250 to 300° C. (523 K to 573 K). However, these species are not readily available in engine exhaust streams. A method for producing these species on-board the vehicle that leads to efficient selective reduction of NOx over the entire desired exhaust temperature range is desireable.
It has been reported that the peak temperature for NOx conversion using diesel fuel can be reduced by injecting the diesel fuel into a region upstream of the catalyst that has been preheated to 400° C. (673 K) while maintaining a lower catalyst temperature. Peak NOx conversion efficiencies have been obtained at catalyst temperatures near 300° C. (573 K) with this method. It was believed that the diesel fuel was partially oxidized at 400° C. (673 K) to form species that were more effective for HC-SCR than diesel fuel by itself. The drawback to this method, however, is that supplemental heating of the exhaust may not be suitable for practical engine exhaust systems on a vehicle. Ignition improvers, when added to diesel fuel, have been shown to lower the autoignition temperature by lowering the temperature where diesel fuel oxidation begins to take place. A method to take advantage of this lower oxidation temperature to prepare partial oxidation products for use in exhaust HC-SCR can be useful.
Specific lean-operating engine configurations may employ oxidation catalytic devices immediately downstream of the engine to manage exhaust emissions related to hydrocarbons and particulate matter, and accommodate fuel sulfur. An oxidation catalyst readily oxidizes aldehydes. Any aldehydes produced in engine in-cylinder burning are oxidized in the oxidation catalyst and are unavailable for NOx reduction in an HC-SCR device.
Thus, there is a need for a method and system to facilitate hydrocarbon-selective reduction of exhaust gas NOx in an aftertreatment system for an engine operating lean of stoichiometry wherein the aftertreatment system includes an oxidation or other catalytic device upstream of the HC-SCR device, while addressing issues related thereto.